Devices of this kind are used, for example, for ultra-fast time resolved spectroscopy. Mode-locked lasers are usually applied as light sources in this connection. So-called pump-probe techniques are used for measuring and investigating the time progression of physical or chemical processes. Such techniques have triggered remarkable progress in various fields of science and technology. Worth mentioning are studies on relaxation dynamics in solid-state bodies, liquids, and gases, as well as time-resolved terahertz spectroscopy and signal analysis in optical communications technology. In synchrotron radiation sources, mode-locked lasers are utilized as light pulse sources in order to control generating of electron bundles in terms of time and also to analyse the time-related behaviour of electron, UV-light or X-ray pulses. All these applications have in common that the arrival times of light pulses must be precisely controllable in an interactive center of the relevant experiment. In most cases, the arrival times and/or the time intervals between consecutively arriving light pulses must be variable within a certain interval in order to be able to scan the time progression of the physical, technical or chemical process to be investigated.
It is known to generate consecutive light pulses having an adjustable time interval by means of a single light source whose light beam is split into two partial beams and reunited again, with a delay distance of variable length being situated in one branch. The variable time interval between the light pulses according to this approach results from the different run-times in the branches fo such an interferometer. The variable length is usually realized by means of an electromechanically moved mirror. A disadvantage lies in that the mirror must be moved over the full way that corresponds to the variation of the time offset. This required large stroke conflicts with a fast movement of the mirror. Hence, the mirror movement is relatively slow and the time interval between light pulses can only be varied accordingly slowly. This entails undesirably long scanning times. Another disadvantage is that the mechanical mirror adjustment is susceptible to misadjustments. To define the time axis of a relevant experiment, the mirror position must be precisely determined for each value of the time offset. Moreover, due to divergency of the light beam, the mirror movement involves an undesired variation of the beam diameter.
To overcome these drawbacks, the so-called ASOPS technique has become known (“Asynchronous Optical Sampling”). According to this technique, two light sources are utilized which emit periodical sequences of light pulses, with the light pulse sequences being superimposed in the interactive center of the relevant experiment. The light pulse sequences of both light sources have a time offset that grows from light pulse to light pulse, and this time offset comes about due to the fact that the repetition frequencies of the light pulse sequences of the two light sources are slightly different. Accordingly, the time offset between the light pulses of the two light sources brushes over the full time interval between two consecutive light pulses from either light source, which means the complete time interval matching the inverse repetition frequency, until the light pulses of both light sources coincide again. Then the process starts all over again. Against this background it is a disadvantage that the scanning range of the ASOPS technique for most practical applications is much too large. The reasons is, as outlined above, that the time offset between consecutive light pulses constantly varies between zero and the full time interval between the light pulses of one of the light pulse sequences. For example, if the repetition frequency of light pulse sequences amounts to 100 MHz, the time offset of the light pulse sequences automatically varies between 0 and 10 ns. However, a scanning range of 10 ns is not needed in practice. For most applications, e.g. for time-resolved spectroscopy, a variable time offset of some 10 ps is absolutely sufficient in view of the time scale of the dynamics investigated. It means that with the ASOPS technique, no sensible measuring data can be obtained for the most part of the measuring period (over 90%). Consequently, the ASOPS technique has a significant disadvantage in that for generating light pulse sequences, one has to utilize light sources whose repetition rates at least amount to one gigahertz. Only in this way is it possible to achieve adequate time resolution with practical scanning rates at the same time for most applications.
Known from DE 20 2008 009 021 U1 is a device of the kind mentioned above, in which a control signal is formed within a control loop from the light pulse sequences by means of a phase detector, with the control loop comprising a controlling element that generates an actuating signal which influences the repetition rate of the light pulse sequence of either light source. By changing the repetition sequence, i.e. the reference variable, a rushing-ahead and/or trailing-behind of the light pulse sequence of the one light source versus the light pulse sequence of the other light source is generated in a well aimed manner. The controlling element sets the repetition rate such that the desired phase value, i.e. the desired time offset, is set. The prior art device has an advantage in that the time offset between the light pulse sequences can nearly arbitrarily be preset. The scanning range, i.e. the range over which the time offset is varied, e.g. in a pump-probe experiment, can be preset arbitrarily. This implies an advantage over the conventional ASOPS technique, in particular with regard to an adaptation of the scanning range to the requirements of a given application. The scanning range and the choice of measuring points, i.e. of the time offset values used in the measurement, are not defined and set by principle, but freely definable.
But controlling the repetition rate of either light source via the control circuit as per DE 20 2008 009 021 U1 also has some disadvantages. For example, the time resolution in varying the time offset is determined by the properties of the control loop. The scanning speed, i.e. the velocity in varying the time offset, is restricted by the bandwith of the control circuit. With fast pump-probe experiments (e.g. in time-resolved terahertz spectroscopy), a scanning speed in a range of 1 kHz is striven for, i.e. the entire scanning range is to be scanned periodically with this frequency. With such a high scanning speed, signal distortion in substantial extent occurs due to the properties of the control circuit. One result hereof is that the time axis of the experiment, i.e. the exact time interval between each single pump-and-probe light pulse cannot be reconstrued any longer precisely. In phase modulation according to the prior art approach, i.e. based upon two light pulse sequences coupled to each other via a phase control circuit, in other words, an undesired interaction occurs between modulating and controlling. The control circuit can follow a modulation of the reference variable only far below the control bandwidth. As modulation frequency and amplitude rise, signal distortion occur. Alternatively, if modulation is directly chirped on the actuator element, the control circuit tries to counteract it. Depending on modulation frequency, i.e. scanning speed, this happens with a different phase. The induced distortion of modulation varies from a counter-control. (below the control bandwidth of the control circuit) to co-modulation, i.e. an intensification of modulation above the bandwidth. In the transient range, controlling causes distortion of modulation which complicates reconstructing the relative phase and/or the relative time offset between the light pulses and thus the necessary calibration of the time axis of the relevant pump-probe experiment, or, depending on demanded accuracy, makes this even impossible. Another drawback lies in the fact that control with the prior art device may induce additional broadband disturbances due to the non-linear response from the control circuit or actuator element, which means that interfering harmonics of the modulation frequency or noise are generated.